In the present study we investigated if the lack of functional cystic fibrosis transmembrane conductance regulator (CFTR) expression influences the paracellular permeability of model bronchial epitheliums 16HBE14o- and CFBE41o- (homozygous for the DF508 mutation) grown to confluence on permeable support. Stimulation of CFTR by cAMP produced in 16HBE14o- a dramatic decrease of transepithelial electrical resistance (TER) within 5 min. This effect was concomitant with a significant decrease in the paracellular permeability of the marker molecule ¹⁴C-mannitol. cAMP induces phosphorylation of Claudin-5, an important component of the tight junctions and as a consequence increases the barrier function. In contrast cAMP induces in CFBE41o- monolayers an increase of TER and the paracellular permeability is reduced slightly. We found that under unstimulated conditions 16HBE14o- monolayers show a two times higher paracellular permeability compared to CFBE41o- monolayers. It was reported that that the AMP-activated protein kinase (AMPK) is up-regulated in DF508 expressing cell lines. AMPK facilitates the assembly of tight junctions and therefore increase the barrier function. Due to the high AMPK activity in CFBE41o- cells tight junctions permeability is already low and therefore stimulation with cAMP had only a minor effect on paracellular pathway. In wtCFTR expressing bronchial epithelium paracellular permeability is high under basal conditions allowing passive osmotic adjustment of the airway surface liquid (ASL) by salt and water movement through the paracellular pathway. Under CFTR stimulating conditions (cAMP) bronchial epithelium secretes actively salt and water into apical side. Simultaneously the paracellular permeability is reduced to avoid reflux of salt and water to the basal side. In CF epithelia the paracellular permeability is low under basal and stimulating conditions due to the abnormal high AMPK activity. As a result no passive osmotic adjustment of the ASL can occur. In summary, our data show that airway epithelium lacking functional CFTR expression is unable to control the paracellular transport.